On 10/23/2016 04:57 PM, Toby Thain wrote: > On 2016-10-23 2:50 PM, shadoooo wrote: >> Hello, >> surely the old transceivers are the most compatible solution, however >> you >> still need to convert the voltages back and forth... >> Plus the solution is not the cheaper, and a little uncomfortable too, as >> you need to find these old chips, hoping not to buy fake chinese >> duplicates >> (it happened to me more time unfortunately). >> >> So I was searching a solution with modern components, but not using >> components too much specific and difficult to be found. >> >> As we need 3.3v logic, but able to work in 5v bus, I'm thinking about 5v >> tolerant standard logic as TI LVC or LVT. >> The problem is that there aren't open drain bus transceivers, but the >> problem could be solved simply using input-only and output-only >> components, >> connecting two in parallel but opposite direction on bidirectional pins. >> So identifying one or maybe two codes would be enough for all the >> components needed for the board. >> >> The idea of using bare transistors seems to me too much simple. >> Not that it couldn't work, but it would be almost impossible to >> satisfy all >> the specifications of the bus in this way... unless you use a more >> complex >> circuit with precise current sources and resistors to grant correct >> voltage >> biases, impedances and slew rates, which in the end is a logic >> integrated >> circuit. >> >> Andrea >> > > As an electronics noob, I'm really waiting for somebody to publish > their findings on this, comprehensively, so I can steal their labour. > > Has anyone done so? Is anyone planning to do so? I know that this > topic flares up on the list every 6 months ago in a series of > disjointed posts and observations. The gold is hard to find > (especially for aforesaid noobs). > > --Toby > Toby,
I've watched many flail on this when simple solutions work. They seem to read into the driver/receiver specs a lot of imagined should be x when its simple. The specs are those of Utilogic and TTL of the day. DEC buses relied on two things; pull-ups to assert the high voltage level and a strong pull down to ground at low cost. All of this was developed in the late 60s and early 70s. To add to that the idea of open collector was to prevent damage if two drivers asserted the same line. Also in some cases to allow wired OR logic. We forget that a transceiver in 1970 was the level of complex logic and the only thing more complex was a flipflop as around then the tech was not there for more than a handful of transistors on the die. By time the tech could do more the parts were firmly entrenched and provided the basic bits often needed. DEC always suggested their part for the base reason is they work, there were examples in use to study and internal engineering was expected to! That and if a customer needed DEC support it was easier. I keep my supply of those for board repair. For new boards I make for myself (QBUS or Omnibus) I use bog simple 74LS14, 74LS240, 74LS241, 74LS245 and occasionally a open collector part like 7438. The 3.3 volt problem is modern logic and not modern in conflict. The trick in engineering around that is to keep the interfaces limited to reduce the needed transitions back and forth. Allison Former MilRat
